Effective grounding is defined by the National Electrical Safety Code (NESC) as “ Bonded to an effectively grounded neutral conductor or to a grounding system designed to minimize hazard to personnel and having resistances to ground low enough to permit prompt operation of circuit protective devices."
This method of grounding ensures the safe, reliable and effective operation of Rocky Mountain
Power’s electric system.
Rocky Mountain Power is committed to operating its electric grid in a safe and reliable manner.
Ungrounded systems are susceptible to elevated voltage levels, especially during phase-toground short circuit events, which can lead to equipment failure and prevent the safe and reliable operation of the system.
Rocky Mountain Power’s distribution system is effectively grounded, as is the majority of the systems across North America. During fault conditions, an effectively grounded system in addition to neutral overcurrent protection helps prevent overvoltage issues by maintaining voltage levels within acceptable levels as published by the American National Standards
Institute (ANSI). This method ensures that generators, consumers and company equipment are protected from impacts caused due to short circuit events. Other methods can also ensure coordination between the system’s protective equipment and the generator’s equipment however they are generally more expensive alternatives for the customer. As a result, Rocky
Mountain Power suggests effective grounding measures as the lower cost option to ensure proper system coordination.
Rocky Mountain Power requires interconnection customers to follow the effective grounding requirements described in the Protection & Control section of its Distributed Energy Resources
Interconnection Policy 138 . For net-metering customers, the company requires customers to follow requirements outlined in the brochure “Connecting to Rocky Mountain Power’s electrical system” .
In addition to company-specific requirements, Rocky Mountain Power follows IEEE 1547, UL
1741, and ANSI C84.1 as well as other national, state and local jurisdiction rules.

With approval from Rocky Mountain Power, effective grounding may not be required when particular criteria are met, which would preclude the possibility of temporary overvoltages.
Based on IEEE 1547 guidelines in addition to Rocky Mountain Power’s policy, some criteria include:
 The DER is connected on a single-phase distribution transformer, and is connected line-to-neutral,
 The DER is connected using a three-phase, three-wire configuration,
 When the DER peak power on the primary distribution system aggregates to less than
10% of the local system’s minimum load (for solar interconnections, minimum daytime load), then the load will likely be sufficiently large to limit temporary overvoltages,
 Installation or existence of certain transformer configurations might be exempted based on interconnection study results as determined by Rocky Mountain Power’s
Protection & Control group.
Although more common on larger generation interconnections, effective grounding can be required even on DER systems less than 25 kilowatts. Individual circuits behind a protective device on a distribution line can have very low minimum loads, thus necessitating effective grounding regardless of the size of the interconnected DER.
A simple example is a distribution circuit that has a peak load of 8,000 kVA and a daytime light load of 1,500 kVA. In this case, the limit of distributed solar generation that can be installed without effective grounding is 150 kVA (10% of light load). If 140 kVA of solar generation already exists and a customer applies for 10 kVA of solar generation, that applicant will need to have effectively grounded generation.
The applicant will not be able to determine whether the light load threshold is already exceeded or will be exceeded with a given application. Applications are addressed in a queue (first in, first out). It is conceivable that a 20 kVA application could be approved without effective grounding immediately before a 10 kVA application is reviewed and found to require effective grounding (or other improvements). Rocky Mountain Power uses the queue to ensure fair treatment of all applications.
Currently UL 1741 and IEEE 1547 are in the process of developing safety and testing procedures for inverters with advanced functionalities (also called “smart inverters”) wherein the generators can limit the interactions between the generator and any system short circuit events.
As the technology advances, changes to requirements are likely to occur. However, due to the standards (as well as the technology) being in a developmental stage, Rocky Mountain Power is not in a position to comment on effective grounding requirements for smart inverters.

The system directly below illustrates a distribution system with the service transformer connected wye on the primary voltage side.
SERVICE TRANSFORMER
PRIMARY VOLTAGE
NEUTRAL
GENERATOR
51
G OVERCURRENT, CO-9
RELAY OR
EQUIVALENT
The system below illustrates a distribution system with the service transformer connected delta on the primary voltage side.
SERVICE TRANSFORMER
PRIMARY VOLTAGE
GENERATOR
NEUTRAL
51
G
Grounding Bank